Pacemakers for generating artificial stimulating pulses for the heart, and which may be implanted in the body, are well known. Originally the electrical circuitry for such pacemakers was of analog design, but in recent years digital circuitry has been also employed. A digital approach to pacemakers has led to the evolution of programmable pacemakers--pacemakers having parameters such as pulse rates which are adjustable (programmable) once the pacemaker has been implanted. The programs can be changed from outside the patient's body by appropriate signal transmission to the implanted pacemaker and without surgery. Programmable pacemakers are described in, for instance, British Specifications Nos. 1,385,954 and 1,398,875. Such pacemakers have circuitry to detect and decode signals transmitted outside the body and alter the program accordingly. In British Specification No. 1,385,954 (claiming priority based on U.S.S.N. 141,694, in turn a parent of U.S. Pat. No. 3,805,796 to Tenz) the programming is accomplished by means of a magnetic field which is sensed by a magnetic reed switch; the opening and closing of the switch providing programming pulses to a program store. In British Specification No. 1,398,875 (based on U.S. Pat. No. 3,833,005 to Wingrove) the programming is by means of radio frequency transmission and reception.
Many pacemakers are of the demand type--that is they only supply a stimulating pulse to the heart when a natural heart beat is absent. To accomplish this, demand pacemakers have means for sensing the presence or absence of natural heart beats and for actuating the stimulating pulse as appropriate.
It is desirable with a demand pacemaker that the stimulating pulses are issued only when really needed by the heart, and that the latter is given the opportunity of functioning as naturally as possible. One approach to providing this desirable property has been to provide the implanted pacemaker with a fixed hysteresis function for the pacing rate, so that, after each natural heart beat detected which inhibits a stimulating pulse, a slight delay occurs before the next stimulating pulse is generated. Hysteresis is therefore the characteristic of a pacemaker whereby the period of time from a natural heart beat to the next pacing pulse is longer than the period between two successive pacing pulses. This hysteresis, which essentially involves the pacemaker switching over to issuing one stimulating pulse at a slower rate after one or more natural beats has arisen, is of particular use in that it avoids competition between natural heart beats and artificial stimulating pulses, and hence reduces current drain on the pacemaker. It is desirable to allow as many natural beats to arise normally without any stimulation of the heart being provided: by supplying a hysteresis function, a greater opportunity is being given for the natural beats to continue without an artificial pulse being generated.
Not all cardiac specialists agree that a hysteresis function is universally desirable for all pacemakers (see for example, The American Journal of Cardiology, 38, p. 685-688 (1976)), and with currently available pacemekrs this entails a decision on the medical personnel whether or not a pacemaker to be implanted should be one with or without hysteresis in its circuitry. To change this decision entails replacing the implanted pacemaker and hence surgery.